Until recently, it had been desired to produce synthetic fibers to more or less match natural fibers, for supply to the textile industry for manufacture into fabrics and garments. So commercial synthetic fibers have been historically made and used for apparel at fineness levels near those of natural fibers. More recently, so called "subdenier" polyester filaments have been available commercially at finer dpfs (deniers per filament less than about 1) but performance and availability of such fine dpf cut fibers (for staple spun yarns) has been limited. This is because conventional spinning and handling technologies have been inadequate for economic production of this subdenier staple, and such fine fibers have been available commercially by expensive routes, such as bicomponent spinning, or other low productivity processes, that are not economically attractive for making commodity staple fiber. Routes to make such fine deniers have been tried, such as (a) to split side-by-side conjugated yarns, (b) to dissolve a sea component out of a sea-island conjugated yarn, (c) to flow-draw freshly-extruded undrawn filaments ("super-stretching") to reduce denier, followed by neck drawing, (d) to use conventional spinning and drawing processes, and (e) to use high speed spinning to obtain oriented fine filaments. Processes (a) and (b) require special spinning devices and are very expensive. It is difficult to produce subdenier filaments using processes (c) and (d) due to spinning and/or drawing breaks and the denier variation has been excessive in practice. The high speed spinning process (e) can be used to produce a fine filament, but this route gives fibers of lower tenacity, lower Young's modulus and higher elongation than fibers prepared by the conventional split low speed spinning and drawing techniques. Further, high speed spinning is not compatible with high cell throughputs or piddling, a step that is necessary for a high throughput split process, i.e., separate spinning and drawing processes. In addition, the low spinning cell extrusion rate is not economical for a staple process.
The present invention is concerned with the preparation of uniform, fine fibers at high productivity using the basic elements of a conventional split staple process, so the staple can then be used in staple yarn processing systems. It is known by those skilled in the melt spinning art that when the denier per filament (dpf) is reduced, the need for polymer uniformity and process control becomes extremely critical and high quality filaments cannot be produced at the same productivities and yields as normal dpfs using conventional spinning. Attempts to manufacture subdenier filament by conventional spinning technology have resulted in filament breakage in the spinning process. So it has been necessary to reduce the amount of polymer extruded or increase the number of filaments per spinneret. When the numbers of filaments are increased, filament uniformity has deteriorated and breakage has occurred. When the amount extruded has been reduced without increasing the number of filaments, a larger number of spinning cells has been required, and so the process has been economically less attractive. In addition, conventional spinning methods have not produced satisfactory fine dpf fibers from difficult-to-spin polymers, e.g. of low viscosity, so fine denier fibers of low viscosity polymers have not been commercially available (from conventional spinning techniques).
To summarize, previous polyester filament manufacturing techniques that have been disclosed in the art have been directed to fibers not suitable for staple, or to fibers with poor uniformity, or the processes used have been expensive and/or have had low productivity.
An object of the present invention is to provide fine, uniform filaments that can be spun at high cell throughputs and be drawn to uniform subdeniers and converted to staple that is suitable for apparel uses by a process that is economically attractive. Another objective is to provide a low viscosity polyester subdenier fiber that is suitable for specialty apparel end-uses.